Absorption spectra of crystalline thiophthene

The near ultraviolet absorption spectra of crystalline thiophthene have been measured in polarised light at temperatures down to 4–6 K. The spectra show that only the Davydov component located at higher energies remains broad at low temperature. The nature of this broadening is discussed in terms of downward inter-band phonon scattering from the upper optical level by a librational phonon. A comparison of experimental data and theoretical calculations of energy levels and polarisation ratios based on the dipole—dipole approximation has been carried out, and the direction of polarisation of the π* ← π electronic transition which lies in the thiophthene molecular plane has been defined.